To reduce the amount of CO2 emission in view of global environmental conservation, an improvement in the fuel consumption of automobiles by reducing the weight of automobile bodies while maintaining the strength of automobile bodies is always an important issue in the automobile industry. In order to reduce the weight of automobile bodies while maintaining the strength thereof, it is effective to reduce the thickness of a steel sheet used as a material of automobile parts by increasing the strength of the steel sheet. Meanwhile, many of automobile parts composed of steel sheets are formed by press working, burring working, or the like. Therefore, it is desired that a high-strength galvanized steel sheets used as materials for automobile parts have excellent d formability in addition to desired strength.
In recent years, high-strength galvanized steel sheets having a tensile strength (TS) of 1180 MPa or more have been increasingly used as materials for automobile body frames. High-strength galvanized steel sheets are required to have good bendability, good ductility, and in particular, good uniform elongation for the formation thereof. Parts composed of high-strength galvanized steel sheets are required to have high yield strength and high impact resistance in view of crashworthiness, and it is extremely important to achieve all these properties. Various high-strength galvanized steel sheets have been developed under these circumstances.
Patent Literature 1 discloses a high-strength galvanized steel sheet having good bendability owing to the control of precipitates and a technology, as a method for producing thereof, for controlling the cooling rate of molten steel prior to solidification, an annealing temperature during annealing, and a subsequent cooling rate.
Patent Literature 2 discloses a high-strength galvanized steel sheet having good ductility and good bendability owing to the control of the balance between Si and Al, retained γ, and the Vickers hardness of portion directly below a surface, and a technology, as a method for producing thereof, for controlling a finishing temperature, a coiling temperature, an annealing temperature range, a cooling rate after annealing, a cooling stop temperature, and a cooling stop holding time.